Grain oriented electrical steel sheets mainly used as iron cores of electric appliances such as transformers are required to have excellent magnetic properties, in particular, low iron loss properties. There have been mainly employed in this regard, as indices of magnetic properties, magnetic flux density B8 at magnetic field strength: 800 A/m and iron loss (per kg) W17/50 when a grain oriented electrical steel sheet has been magnetized to 1.7 T in an alternating magnetic field of excitation frequency: 50 Hz.
To reduce iron loss of a grain oriented electrical steel sheet, it is important to subject the steel sheet to secondary recrystallization annealing so that secondary recrystallized grains are accumulated in {110}<001> orientation (or Goss orientation) and to reduce impurities in the product.
However, there are limitations in controlling crystal orientation and reducing impurities in terms of balancing manufacturing cost, and so on. Therefore, some techniques have been developed to introduce non-uniformity to the surfaces of a steel sheet in a physical manner and artificially reducing the magnetic domain width to reduce iron loss, namely, magnetic domain refining techniques.
For example, JP 57-002252 B proposes a technique for reducing iron loss by irradiating a final product steel sheet with a laser, introducing a linear, high dislocation density region to the surface layer of the steel sheet and thereby reducing the magnetic domain width.
In addition, JP 06-072266 B proposes a technique for controlling the magnetic domain width by means of electron beam irradiation.
To perform magnetic domain refining treatment such that it is effective in reducing iron loss, it is necessary to introduce relatively large thermal energy to a surface of a steel sheet. However, a problem arose when such large thermal energy was introduced to a surface of the steel sheet, where the steel sheet suffered warping toward the surface on which the strain-introducing treatment had been performed.
Once warping occurs, the steel sheet may possibly experience a degradation in handling ability when assembled as transformers or the like, deterioration in hysteresis loss due to its shape, deterioration in hysteresis loss caused by the elasticity strain introduced when the steel sheet is assembled as transformers or the like, and so on. This is considered significantly disadvantageous in terms of both manufacture and properties.
It could therefore be helpful to provide a grain oriented electrical steel sheet having sufficiently low iron loss and having less conventionally-concerned warpage of the steel sheet effectively even after the steel sheet is subjected to artificial magnetic domain refining treatment, where strain-introducing treatment is conducted with high energy so that an iron loss-reducing effect can be maximized.